With the increasing need to develop alternative forms of energy to address the problems of pollution and the dependence on oil, fuel cells have received increasing attention as a premier source of clean and quiet power. However, due to the costs associated with the materials that go into making these fuel cells, they are not economically feasible for use in many applications.
There are several competing fuel cell technologies. These technologies include alkaline fuel cells, proton exchange membrane (PEM) fuel cells, etc. Although each technology possesses certain advantages over the other, alkaline fuel cells offer the potential for higher power capability, high operating efficiency and lower cost of manufacture.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between hydrogen and hydroxyl ions (OH−) present in the electrolyte that form water and release electrons:H2+2OH−→2H2O+2e−.
The oxygen reduction reaction typically takes place via a 2 step reaction, each step providing a 2 electron transfer. In other cases, such as with the use of pure platinum, it has been reported that oxygen reduction may be accomplished via a single step, 4 electron transfer. However, once the platinum is exposed to an impurity, the direct 4-electron transfer may not be realized.
The consequence of the two-step reduction process is the formation of peroxyl ions:O2+H2O+2e→HO2−+OH−.  (1)HO2−+H2O+2e→3OH−.  (2)Overall: O2+2H2O+4e→4OH−.
Although the final reaction is ultimately hydroxyl ion formation, formation of intermediate species can be very problematic. Peroxyl ions are very reactive and can oxidize many materials. In a porous oxygen diffusion electrode where the electrochemical reactions are taking place at the surface, the formation of peroxyl ions becomes detrimental to the performance of the fuel cell. The pores at the electrode surface provide sites for oxygen reduction as long as the pores are accessible to the electrolyte. Once the oxygen reduction takes place, peroxide formation as an intermediate of the product of reaction occurs within the pores. Since the pores are not through-hole pores, the peroxide has no way to escape except by diffusion into the bulk. Bulk diffusion can be a rather slow process. During this time, peroxide can (1) oxidize the teflonized carbon, (2) decompose and form gas bubbles that can block the pores causing a loss of surface area, and (3) react with the active catalyst material to destroy its character. All three of these factors can lead to gradual flooding and a loss of performance within the oxygen electrode. Thus, the effect of peroxide formation/reaction can be observed as a gradual increase in polarization and a sudden loss of performance.
Catalysis primarily occurs at certain favorable locations called active sites. It has generally been taught that these active sites can be altered to increase the performance of catalysis. For example, as described in U.S. Pat. No. 5,536,591 to Fetcenko et al., entitled Electrochemical Hydrogen Storage Alloys For Nickel Metal Hydride Batteries, catalyst type, state, size, proximity, porosity and topology are several factors that can be altered to engineer new catalysts. The '591 patent and its progeny, demonstrate that small sized catalytic particles, such as 50 to 70 angstroms, can be formed in an oxide support within a very small proximity to one another, such as within 2 to 300 angstroms. Such catalysts have revolutionized the NiMH battery industry.
Catalysts can be either supported or non-supported. Supported catalysts are those that have the catalyst fixed to a carrier matrix, while non-supported catalysts are those that are free from any carrier matrix. Examples of supported catalysts include metals supported on carrier matrices such as refractory oxides, carbon, or silicon dioxide. Examples of non-supported catalysts include spongy metal catalysts, such as Raney nickel, spinels, or other fine metal powders, such as platinum, gold, palladium, silver, etc. There presently exist a multitude of supported catalysts, which have been designed for specific uses. Below are several examples of these catalysts.
Catalysts have been developed for the treatment of wastewater. See for example, U.S. Pat. No. 4,670,360 to Habermann et al., entitled Fuel Cell. Habermann et al., which discloses a fuel cell having an activated carbon-containing anode and an activated carbon-containing cathode for use in the oxidative treatment of wastewaters containing oxygen or oxygen containing compounds. The patent describes using graphite and active carbon as a carrier support.
Catalysts have been developed for the cathodic evolution of hydrogen in electrolysis plants. See for example, U.S. Pat. No. 3,926,844 to Benczur-Urmossy, entitled Catalysts For The Cathodic Hydrogen Development. Benczar-Urmossy, which describes depositing X-ray-amorphous boride compound of nickel, cobalt or iron on a supporting structure. The compound is deposited from an aqueous solution having metallic ions such as nickel ions, cobalt ions, or iron ions, with a complexing agent and a water-soluble borate or borazane at a temperature of below 60° C.
Catalysts have been developed for use in hydrocracking gas oil. See for example U.S. Pat. No. 4,686,030 issued to Ward, entitled Mild Hydrocracking with a Catalyst Having A Narrow Pore Size Distribution, which discloses metal oxide catalysts supported on a calcined oxide support. The catalyst may be made by extruding a gamma alumina-containing material through a die, drying the alumina, and breaking the alumina into pieces to form the support. The support is then impregnated with nickel nitrate hexahydrate and ammonium heptamolybdate dissolved in phosphoric acid, dried and calcinated.
Catalysts have been developed for use in air cathodes for electrochemical power generation. See for example, U.S. Pat. No. 6,368,751 B1 to Yao et al., entitle Electrochemical Electrode For Fuel Cell. Yao et al. discloses an electrochemical cathode including a porous metal foam substrate impregnated with a mixture of carbon, CoTMPP, and Teflon.
A number of techniques for making catalysts have also been developed. These techniques include impregnating, coating, or simply mixing metal powder in with a support. See for example, U.S. Pat. No. 4,113,658 to Geus, entitled Process for Homogeneous Deposition Precipitation of Metal Compounds on Support of Carrier Materials, which discloses a method of making supported catalysts by precipitating a metal salt solution onto a carrier matrix.
Another technique for making catalyst was taught by Ovshinsky et al. in U.S. Pat. No. 4,544,473, entitled Catalytic Electrolytic Electrode, which describes making amorphous, catalytic bodies by various deposition techniques.
Catalysts can also be made by depositing organometallic catalysts onto a support and then removing the organic material by heating at a relatively high temperature. See for example, U.S. Pat. No. 4,980,037, entitled Gas Diffusion Cathodes, Electrochemical Cells and Methods Exhibiting Improved Oxygen Reduction Performance, to Hossain et al.
Because the design of each catalyst is often the limiting factor to its ultimate end use, there continues to be a need for new and improved catalysts and ways for making them. Furthermore, if fuel cells are to become cost competitive with other forms of power generation, high efficiency, low cost catalysts for use in these fuel cells needs to be provided.